The three main components of a computer system: CPU, memory, and input/output devices
1. Bus
①Each input/output device consists of two parts, one part is the
I/O controller
, including most of the interface circuits; the other part is the
device itself
. The I/O controller is usually integrated in the motherboard, and sometimes in the form of a plug-in card in the empty slot of the motherboard.
② The task of the I/O controller is to control its input/output devices and process the access signals on the bus.
③ When the program needs data on the disk, it tells the disk controller a command, and then issues seek and other commands to the disk drive. A group of commands usually consists of one or several words, and the I/O controller can complete reading and writing to the memory without CPU intervention. This method is
called
Direct Memory Access .
④The bus should not only be used by the I/O controller, but also used by the CPU to obtain instructions and data. So, a chip called a
bus arbiter
decides who gets to use it. In general, input/output devices have higher priority than CPU, because disks and some other mobile devices cannot be suspended, and forced suspension may result in data loss.
⑤ Therefore, when there is no input/output device request, the CPU can monopolize the entire bus. And when the input/output device requests, the device will issue a bus request when needed, and be authorized to use the bus. This process is
bus stealing
.
⑥ ISA bus (Industry Standard Architecture, industry standard structure bus). (When the machine level will be mentioned later, ISA stands for the instruction system layer, remember not to be confused with the ISA bus)
⑦EISA bus (Extended ISA, extended ISA bus)
⑧PCI bus (Peripheral Component Interconnect, external device component interconnection)
⑨ PCIe bus (PCI Express, extended PCI)
2. Terminal
①Keyboard: There are roughly two types of keyboards: mechanical and electromagnetic.
当按键被按下的时候，产生一个中断请求，计算机中的中低端响应程序被启动 ，并由它从键盘控制寄存器中读出被按下的键位的序号。
②触摸屏：触摸屏主要分为三类：红外型、电阻型和电容型。
红外型：在屏幕的左边沿和上边沿安装以后红外线的发射装置，对应的在右边沿和下边沿安装有接收装置，当手指或笔这种不透明的物体阻挡了一条或者多条光线时，操作系统就能计算出笔尖的（x,y）坐标。
电阻型：这种触摸屏由两层构成，每层都有平行的导线构成，横着一层，书这一层，两者不接触。上面一层具有一定的弹性，当手指或者其他物体挤压屏幕时，上层的一条或者多条导线会接触到下层垂直方向的导线。这样，连接在到线上的感应器能判断出屏幕上那部分区域按下去了。
电容型：前两种基本只支持单点触控，而现在大多手机都才用多点触摸屏，使用
投射电容触摸屏（projected capacitive touch screen）
。这种屏幕是 由网格状的 垂直布置的 被绝缘体分开的 两个电极组成。当手指触摸屏幕时，被触摸的交叉点的电容会发生变化，这种变化是可以被检测到的。这种电极又是由极细的 透明的 可导电的 铟锡氧化物细丝 组成的，粘合在 薄的 玻璃板的背面一起构成了电容。
③平板显示器
a.第一台计算机显示器使用的是阴极射线管显示器（Cathode Ray Tube ,CRT）
b.目前应用最广泛的就是
液晶显示器（LCD显示器，Liquid Crystal Display）
了。液晶是一种胶状的有机分子，可以像液体一样流动，但又有像水晶一样的空间结构，是1888年一位奥地利的植物学家Fridrich Reinitzer发现的。
c.绞合向列型（TN，Twisted Nematic）显示器：这是一种LCD显示器，当没有电场存在的时候液晶分子将顺着槽排列，如下图，这两个槽是互相垂直绞合的。
显示屏的后面是一块水平偏振滤光镜，只允许水平偏振光通过。而显示器前面是垂直偏振滤光镜只允许垂直偏振光通过。